Carburization of ferrous-based shape memory alloys

ABSTRACT

The shape memory effect of an article made from a ferrous-based shape memory alloy is enhanced by carburizing the article&#39;s surfaces.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional patent application Ser. No. 60/642,932 filed on Jan. 10, 2005 for CARBURIZATION OF FERROUS-BASED SHAPE MEMORY ALLOYS, the entire disclosure of which is fully incorporated herein by reference.

BACKGROUND AND SUMMARY

In an article by Kate Ireland entitled “Iron-based Shape Memory Alloys” found at www. uow.edu.au/eng/mm/shapememoryalloys on Nov. 21, 2004, it is indicated that the effectiveness of the shape memory phenomenon in iron-based shape memory alloys “is also enhanced by the creation of an interstitial solution of carbon in austenite.” See, also, Tsuzaki et al., Scr. Metall., Vol. 27, p. 471 © 1992 and Maki et al., Shape Memory Effect in Ferrous Alloys, Proceedings of the Institute of Metals, Nara, Japan, 1986.

In accordance with the present invention, the shape memory phenomenon in iron-based shape memory alloys is enhanced to a degree greater than possible in, or contemplated by, earlier work by surface carburizing articles made from the alloy, while in austenitic form, so as to achieve an increased surface concentration of interstitial carbon. In a preferred embodiment, the article is subjected to low temperature carburization so that carburization is accomplished without substantial formation of carbide precipitates.

Thus, the present invention provides a new process for providing an article formed from an iron-based shape memory alloy having superior shape memory properties, the process comprising carburizing at least a portion of the article's surfaces while in austenitic form so as to achieve an increased surface concentration of interstitial carbon.

In addition, the present invention provides a new article of manufacture comprising a shaped article formed from an iron-based shape memory alloy in austenitic form, the concentration of interstitial carbon in at least a portion of the article's surfaces being greater than in its body.

DETAILED DESCRIPTION

Ferrous-Based Shape Memory Alloys

Shape-Memory Alloys are metals that, after being strained, revert back to their original shape when heated to an appropriate temperature, referred to as the “reverse transformation temperature.” See, Chapter 1 of Otsuka et al., Shape Memory Materials, Cambridge University Press, © 1998. Ferrous-based shape-memory alloys contain a substantial amount of iron. Normally in the austenitic phase, they undergo martensitic transformation (i.e. transform into the martensite phase) when strained and then return to the austenitic phase when heated to their reverse transformation temperatures. See, Chapter 5 of Otsuka et al.

Many ferrous-based shape-memory alloys are known. See, Chapter 5 and especially Table 5.1 of Otsuka et al. See, also, U.S. Pat. No. 4,933,027 to Yutaka Moriya et at. and U.S. Pat. No. 5,199,497 to Richard Ross, the disclosures of which are incorporated herein by reference.

For example, Japanese Patent Provisional Publication No. 61-201,761 dated Sep. 6, 1986, describes ferrous-based shape-memory alloys composed of:

-   -   20 to 40 wt. % Mn,     -   3.5 to 8.0 wt. % Si,     -   at least one element selected from the group consisting of:         -   up to 10 wt. % Cr,         -   up to 10 wt. % Ni,         -   up to 10 wt. % Co,         -   up to 2 wt. % Mo,         -   up to 1 wt. % C,         -   up to 1 wt. % Al,         -   up to 1 wt. % Cu, and     -   the balance being iron and incidental impurities.

Meanwhile, U.S. Pat. No. 4,933,027 to Yutaka Moriya et at. describes ferrous-based shape-memory alloys composed of:

-   -   from 5.0 to 20.0 wt. % Cr,     -   from 2.0 to 8.0 wt. % Si,     -   at least one element selected from the group consisting of:         -   from 0.1 to 14.8 wt. % Mn,         -   from 0.1 to 20.0 wt. % Ni,         -   from 0.1 to 30.0 wt. % Co,         -   from 0.1 to 3.0 wt. % Cu, and         -   from 0.001 to 0.400 wt. % N,     -   with the balance being iron and incidental impurities,     -   where Ni+0.5Mn+0.4Co+0.06Cu+0.002N.≧0.67(Cr+1.2Si)-3.         Carbon-Enhanceable Ferrous-Based Shape Memory Alloys

As indicated in the Ireland, Tsuzaki et al. and Maki et al. articles mentioned above, the shape memory effect of at least some of these ferrous-based shape memory alloys can be enhanced by increasing their concentrations of interstitial carbon atoms, i.e., carbon atoms not chemically bound as carbides to other elements present. For convenience, such alloys will be referred to in this disclosure as “carbon-enhanceable ferrous-based shape memory alloys” or “carbon-enhanceable alloys” for short.

Normally, increasing the interstitial carbon content of these alloys is done by alloying, i.e. by including carbon-yielding materials in the ingredients used to form the alloys. However, there are practical limits to the amounts of interstitial carbon that can be incorporated into such alloys in this way.

Carburization

In accordance with the present invention, additional amounts of interstitial carbon are incorporated into shaped articles made from carbon-enhanceable ferrous-based shape memory alloys while in austenitic form by carburization, i.e. by contacting the article with a carburizing gas at elevated temperature whereby carbon atoms diffuse into the article's surfaces.

Carburization of steels and other ferrous-based alloys has traditionally been done for improving the surface hardness of the alloy. This process, which is known in industry as “case hardening,” is normally done at 1700° F. (950° C.) or above. At these temperatures, and with the steel or other alloy is in the austenitic phase, carbon atoms rapidly diffuse into the article's surfaces. At these temperatures, this diffused carbon forms carbide precipitates, which are specific chemical compounds such as iron carbide, chromium carbide and the like suspended in a matrix of the surrounding metal. Carbide precipitates are very hard, and so the resultant carburized surface or “case” is also very hard. See Stickels, “Gas Carburizing”, pp 312 to 324, Volume 4, ASM Handbook, copyright 1991, ASM International. Nonetheless, at least some of the carbon atoms which diffuse into the article's surfaces remains chemically uncombined and hence present in the carburized surface in interstitial form.

Accordingly, in one embodiment of the present invention, the concentration of interstitial carbon atoms in some or all of the surfaces of shaped articles made from carbon-enhanceable ferrous-based shape memory alloys while in the austenitic phase is increased by traditional case hardening techniques, i.e. by contacting the surfaces to be carburized with a carbon-containing gas at elevated temperature under conditions such that carbide precipitates form in these surfaces.

Low Temperature Carburization

Although carbide precipitates enhance surface hardness, they also can promote corrosion.

In commonly-assigned U.S. Pat. Nos. 6,093,303, 6,165, 597 and 6,547,888 B1, we describe techniques for case hardening stainless steel in which the workpiece is gas carburized below 1000° F. See, also, U.S. Pat. No. 5,792,282, EPO 0787817 and Japanese Patent Document 9-14019 (Kokai 9-268364). The disclosures of all of these documents are incorporated herein by reference. At these temperatures, and provided that carburization does not last too long, the workpiece will carburize with little or no formation of carbide precipitates. As a result, the workpiece surfaces not only become hardened but also retain the inherent corrosion resistance of the stainless steel. Most significantly, comparatively large amounts of interstitial carbon, e.g. 2-12 atomic % and even greater, can be incorporated into the article's surfaces in this way.

In accordance with another embodiment of the invention, the concentration of interstitial carbon atoms in some or all of the surfaces of shaped articles made from carbon-enhanceable ferrous-based shape memory alloys while in the austenitic phase is increased by low temperature carburization, i.e. by contacting the surface to be carburized with a carbon-containing gas at elevated temperature under conditions so that elemental carbon diffuses into these surfaces without substantial formation of carbides. By this approach, the beneficial effects of increasing interstitial carbon atom concentration can be realized without compromising corrosion resistance to any appreciable degree. Moreover, the concentration of interstitial carbon atoms in the article's surfaces can be increased to levels not obtainable at more elevated temperatures.

For convenience, the carburization processes described in this section are referred to as “low temperature carburization.” Note also that, as described in the references cited in this section, it is typically necessary to activate the surfaces of austenitic stainless steels before carburization to enable carbon diffusion to occur.

The Carburized Product

In accordance with the present invention, a shaped article made from a carbon-enhanceable ferrous-based shape memory alloy in the austenitic phase is provided with one or more carburized surfaces having a concentration of interstitial carbon atoms which is greater than the concentration of interstitial carbon atoms in the metal forming the body of the article. As a result, the shape memory effect exhibited by the article as a whole is believed to be even more pronounced than an otherwise identical article without a carburized surface due to the stronger shape memory effect occurring in the article's carburized surfaces.

The amount by which the concentration of interstitial carbon atoms in the article's carburized surfaces should be increased relative to its body can vary widely, and essentially any amount will be effective. Normally, however, the amount interstitial carbon concentration in the article's surfaces should be sufficient to create a noticeable improvement on the shape memory effect exhibited by the article. This can be easily determined by subjecting two otherwise identical sample articles, one with a carburized surface in accordance with the invention and the other without, to the same shape memory cycle of deformation and reverse transformation and measuring the force generated by the two articles during the reverse transformation portion of the cycle. Where this “return” force is greater in the carburized article, the amount of carburization has been sufficient to generate a “noticeable improvement” in the shape memory effect.

The concentration of interstitial carbon atoms in the article's carburized surfaces can also be determined by known analytical techniques including ESCA (Electron Spectroscopy for Chemical Analysis) and X-ray diffraction. Carbon-enhanceable ferrous-based shape memory alloys typically have carbon concentrations on the order of less than about 1 atomic %, more typically less than about 0.5 atomic %. Carburized articles in accordance with the present invention normally will have case hardened surfaces layers on the order of about 10 to 30 microns thick with carbon concentrations of as little as 2 atomic % to as high as 18 atomic % or higher, more typically about 4-12 atomic % or even 6-12, atomic %.

That is to say, the carburized surface layer of increased carbon concentration formed in accordance with the present invention extends from the very outside surface of the article down to a depth which is normally between about 10 to 30 microns. The concentration of carbon at the very surface of the article, as measured by ESCA and/or X-ray diffraction, will normally range from as little as 2 atomic % to as high as 18 atomic %. More typically, this surface carbon concentration will be between about 4-16 atomic %, more typically about 6-12 atomic %. This elevated carbon concentration decreases as the distance from the very surface of the article increases, with the carbon concentration decreasing to the same level as that of the body of the article at depths typically between about 10 to 30 microns. In any event, interstitial carbon concentrations of at least about 2 atomic %, more typically at least about 4 atomic % and even a least about 6 atomic % as measured by ESCA and X-ray diffraction are readily obtainable in accordance with the present invention.

Finally it should be understood that, for the purposes of this disclosure, “shaped article” is intended to refer to an article of any shape other than in the shape in which the bulk alloy is received from the mill. When an alloy is manufactured, a molten mass of its ingredients is poured into a mold and solidified. The ingot so made is then usually formed into a convenient shape for delivery to the customer in bulk. Wire, sheet, rods and bars of various thicknesses and indeterminate lengths are examples. These bulk products are then shaped into useful articles by some type of forming operation which may involve subdividing the bulk product into subsections and then imparting a useful shape to the subsection by cutting, bending, hot and/or cold working, extruding, forging or other metal-working operation. In the context of this disclosure, “shaped article” is intended to exclude the bulk products delivered from the mill but to include any products made therefrom, i.e., any product of commerce which is obtained by imparting a useful shape to a bulk alloy by any type of metal shaping operation.

Although only a few embodiments of the present invention have been described above, many modifications can be made without departing from the spirit and scope of the invention. All such modifications are intended to be included within the scope of the present invention, which is to be limited only by the following claims: 

1. The process comprising carburizing at least a portion of the surface of a shaped article formed from a carbon-enhanceable ferrous-based shape memory alloy in the austenitic phase to produce a concentration of interstitial carbon in the carburized surface portion greater than in the body of the article.
 2. The process of claim 1, wherein carburization is continued until the concentration of interstitial carbon in the carburized surface portion provides a noticeable improvement in the shape memory effect exhibited by the article.
 3. The process of claim 1, wherein carburization is continued until the concentration of interstitial carbon in the carburized surface portion is at least 2 atomic %.
 4. The process of claim 3, wherein carburization is continued until the concentration of interstitial carbon in the carburized surface portion is at least about 4 atomic %.
 5. The process of claim 4, wherein carburization is continued until the concentration of interstitial carbon in the carburized surface portion is at least about 6 atomic %.
 6. The process of claim 1, wherein carburization is accomplish by low temperature carburization.
 7. The process of claim 6, wherein the alloy is a stainless steel.
 8. The process of claim 7, wherein all surfaces of the shaped article are carburized.
 9. A shaped article formed from a carbon-enhanceable ferrous-based shape memory alloy, at least a portion of the surface of the article carrying a carburized surface layer having a concentration of interstitial carbon greater than in the body of the article.
 10. The shaped article of claim 9, wherein the concentration of interstitial carbon in the carburized surface portion is sufficient to provide a noticeable improvement in the shape memory effect exhibited by the article.
 11. The process of claim 9, wherein the concentration of interstitial carbon in the carburized surface portion is at least 2 atomic %.
 12. The shaped article of claim 11, wherein the concentration of interstitial carbon in the carburized surface portion is at least about 4 atomic %.
 13. The shaped article of claim 12, wherein the concentration of interstitial carbon in the carburized surface portion is at least about 6 atomic %.
 14. The shaped article of claim 9, wherein the carburized surface portion is substantially free of carbide precipitates.
 15. The shaped article of claim 14, wherein the alloy is a stainless steel.
 16. The shaped article of claim 9, wherein all surfaces of the shaped article are carburized.
 17. The shaped article of claim 9, wherein the carbon-enhanceable ferrous-based shape memory alloy is in the austenitic phase. 